Hartwell's first SF anthology was compiled for the US Book-of-the-Month Club as one the club's series of World Treasuries. Club editor Clifton Fadiman provides a short foreword.
Hartwell notes in his introduction that, at well over 1000 pages, this is "the largest and most amibitious collection of science fiction from all over the world ever compiled". His selections date "from the modern period", 1939 to present.